PcrA helicase



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What is a Helicase?
Spacefill Helicases are nucleic acid–dependent ATP-ases that are capable of unwinding DNA or RNA  duplex substrates. As a consequence, they play roles in almost every process in cells that involves nucleic acids, including DNA replication and repair, transcription, translation, ribosome synthesis (1).

PcrA a Simple Model for Helicases
PcrA is part of the replication machinery of the Geobacillus stearothermophilusa gram (+) bacteria, This helicase is part of the superfamily I of Helicases. Monomeric protein that is mainly alfa helical has the highly conserved Rec domians. This helicase was reported as a mutation in the gen PcrA from "Stapphylococcus aerous", this mutation was related to a deficiency in the replication of a reporter plasmid.

PcrA Biochemistry
PcrA is has an ATPas activityt which directionality is from 3' to 5' helicase strand separation reaction. The enzyme shows a specificity for the DNA substrate in gel mobility assays with the preferred substrate being one containing both single and double stranded regions of DNA. In contrast to Rep and UvrD from E. coli, there is not evidence for dimerisation of the enzyme using gel filtration, or by crosslinking in the presence of combinations of Mg2+, nucleotides and DNA. Moreover, kcat for ATP hydrolysis is constant over a large range of protein concentrations. Therefore, the protein appears to be monomeric under all conditions tested, including in the structure of two crystal forms of PcrA.

PcrA Helicase Mechanism : The Mexican Wave
Professor Dale B. Wigley' group in 1996-1999 was able to crystalize the intermediate states from PcrA, giving solution to the controversy of what kind of mechanism this helicase has.  Two crystal form of the enzyma, one couple with a 10 mer DNA and a non hydrolizable form of ATP (ATPnP) (pdb id: 3pjr 3pjr, (Enzyme Subtrate Structure) and another a truncated form embebed in sulfate (pdb id: 2pjr 2pjr(Enzyme Product Structure), give a light in a model for how ATP hydrolysis results in motor movement along ssDNA. In the figure below step 1 (top) is the ATP free (product) ssDNA conformation. The DNA bases are labelled arbitrarily. On binding ATP, F626 creates a new binding pocket for base 6. Likewise, F64 destroys an acceptor pocket for base 2, forcing it to move to the position occupied by base 1. After ATP hydrolysis, the grip on base 6 is released. When the Y257 pocket is re-opened due to movement of F64, bases 3-6 can now flip through the acceptor pockets to their new positions. This model predicts that each ATP hydrolysis event will advance PcrA one base along ssDNA.

The link below show a movie with the principal characteristics of this protein as long with the inchworm model. Pcr4 Helicase and Mexican Wave

The Superfamily 1 (SF1)
PcrA share structural domains with the Rec helicases, like UvrD and RepD from E. coli, Superfamily 1 (SF1) helicases are probably the best characterized class, certainly from a structural perspective. All members characterized to date are bona fide helicases and α enzymes. Indeed, from their mode of translocation via the bases it is difficult to envisage how they could translocate along a duplex. However, they can have either A or B directional polarity.

About this Structure
1PJR is a Single protein structure of sequence from Geobacillus stearothermophilus. Full crystallographic information is available from OCA.

Additional Resources
For additional information, see: DNA Replication, Repair, and Recombination

Reference
Crystal structure of a DExx box DNA helicase., Subramanya HS, Bird LE, Brannigan JA, Wigley DB, Nature. 1996 Nov 28;384(6607):379-83. PMID:8934527

Page seeded by OCA on Mon Jul 28 01:38:49 2008

^ Johnson DS, Bai L, Smith BY, Patel SS, Wang MD (2007). "Single-molecule studies reveal dynamics of DNA unwinding by the ring-shaped t7 helicase". Cell 129 (7): 1299–309. doi:10.1016/j.cell.2007.04.038. PMID 17604719. ^ a b "Researchers solve mystery of how DNA strands separate" (2007-07-03). Retrieved on 2007-07-05. ^ Dumont S, Cheng W, Serebrov V, Beran RK, Tinoco Jr I, Pylr AM, Bustamante C, "RNA Translocation and Unwinding Mechanism of HCV NS3 Helicase and its Coordination by ATP", Nature. 2006 Jan 5; 439: 105-108. Anand SP, Zheng H, Bianco PR, Leuba SH, Khan SA. DNA helicase activity of PcrA is not required for displacement of RecA protein from DNA or inhibition of RecA-mediated DNA strand exchange. Journal of Bacteriology (2007) 189 (12):4502-4509. Bird L, Subramanya HS, Wigley DB, "Helicases: a unifying structural theme?", Current Opinion in Structural Biology. 1998 Feb; 8 (1): 14-18. Betterton MD, Julicher F, "Opening of nucleic-acid double strands by helicases: active versus passive opening.", Physical Review E. 2005 Jan; 71 (1): 011904.